The present invention relates to hydrostatic transmissions and control systems therefor, and more particularly, to such transmissions including both a variable displacement pump and a variable displacement motor.
Although the present invention may be used with any transmission of the general type referred to above, it is especially advantageous when used with a variable motor which is controlled by a motor stroking valve such as that shown in U.S. Pat. No. 4,091,617, assigned to the assignee of the present invention, and incorporated herein by reference. The above-incorporated patent illustrates an electromagnetically operated valve to control pump displacement, but a substantially identical valve is now sold commercially by the assignee of the present invention as a motor stroking valve.
One of the primary applications for hydrostatic transmissions in which both the pump and motor are variable displacement is to propel vehicles, such as construction vehicles, which stop and start frequently, and are often subjected to heavy loads. On such vehicles, operation with the motor at maximum displacement provides a high-torque, low-speed mode for handling loads, while operation with the motor at minimum displacement provides a high-speed, low-torque mode, such as when the vehicle is being transported from one location to another.
In transmissions and control systems of the type to which the invention relates, when the main pump controller is put in neutral position, and the vehicle comes to a stop, the motor stroking valve senses relatively low system pressure and destrokes to minimum motor displacement. Upon subsequent start-up of the vehicle, by moving the pump controller away from neutral, the system pressure rises to a relatively high pressure, which indirectly acts on the motor swashplate to gradually move it to the maximum displacement position, which typically requires several seconds until the motor is at maximum displacement. As this is occurring, the system pressure normally rises above the relief valve setting for a short time, thus generating a substantial amount of heat and wasting substantial engine horsepower.